What 5.1 audio can Steam record?

Steam currently only records 5.1 audio from processes launched through Steam on Windows. Other host system audio will be mixed with audio recorded from the game process as stereo audio.

Is 5.1 audio supported when streaming to a Linux or OSX client?

Yes, however you will need to go into the In-Home Streaming settings and select your speaker configuration manually.

Requirements for 5.1 audio on TV

The Steam Client and Steam Link output 6 channel PCM audio when configured for 5.1 surround sound audio. When connected to a TV, the TV or audio receiver need to support 6 channel PCM. Please refer to your device's user manual to check if your device supports this format. It is usually required to use an HDMI cable to connect to your Steam Link, instead of an optical cable, to enable PCM support.